Solvent and ligand effects associated with the Rh(II)-catalyzed reactions of α-diazo-substituted amido esters.

Abstract

We report a detailed investigation into the Rh(II)-catalyzed reactions of 2-alkynyl 2-diazo amido-substituted esters. The distribution of products was found to be dependent on the substituent group on the nitrogen atom, the ligand on the Rh(II) center, and the solvent used. The dominant product obtained from the reaction of 3-(trimethylsilyl)prop-2-ynyl-2-(dibenzylcarbamoyl)-2-diazoacetate (34) with Rh2(OAc)4 in hexane corresponds to an azetidinone derived by CH-insertion of the carbenoid into the neighboring benzyl group. In contrast, the Rh2(esp)2-catalyzed reaction of 34 in CH2Cl2 afforded a 3-oxocyclohepta[c]pyrrole formed by cyclopropanation of the rhodium carbenoid across the aromatic π-bond. Related systems were studied, and CH-insertion into an adjacent alkyl group was found to be the dominant or exclusive pathway. In none of the cases studied was it possible to detect products derived from a carbenoid/alkyne cascade sequence as had previously been found with a series of 2-alkynyl-2-diazo-3-oxobutanoates.